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How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 6)

Release time:

2026-10-20 16:52

How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 6)

Surface defects are a typical challenge encountered when applying UV‑curable resins to difficult‑to‑adhere substrates, manifesting as irregularities such as craters, orange‑peel texture, or residual air bubbles on the coating surface. This issue is particularly critical in applications with stringent aesthetic requirements—such as automotive interiors and appliance panels—where it directly impacts both visual smoothness and protective performance. Because these substrates exhibit low surface energy, the wetting and spreading behavior of the coating differs from that on conventional substrates, leading to unique causes and characteristic manifestations of surface defects. To address the three primary types of surface defects—craters, orange peel, and residual air bubbles—solutions can be pursued through both formulation design and process control.

I. Reducing Shrinkage Cavities

Shrinkage cavities are common surface defects, characterized by the appearance of circular holes on the coating surface. In waterborne systems or formulations containing low‑surface‑tension components, shrinkage cavities can easily form if there is an excessive disparity in surface energy between the coating and the substrate, or if incompatible substances are inadvertently introduced into the coating. Waterborne coatings typically exhibit higher surface tension than solvent‑borne systems; when applied to low‑energy, difficult‑to‑wet substrates, inadequate wetting may lead to localized paint retraction, resulting in shrinkage cavities. Similarly, systems containing low‑surface‑tension additives—such as leveling agents or certain adhesion promoters—can develop localized areas of reduced surface tension if these components are unevenly distributed or poorly compatible with the formulation, thereby triggering shrinkage cavities. Contaminants such as oil, dust, or residues from other coating systems can also serve as initiation points for shrinkage cavities. Once formed, shrinkage cavities not only compromise the coating’s appearance but also reduce local film thickness, potentially exposing the substrate and creating weak spots that undermine protective performance.

When addressing this issue, formulation design can involve adjusting the type and dosage of leveling agents to match the coating’s surface tension with the substrate’s surface energy, thereby enhancing wetting and spreading. In terms of process control, the coating environment should be kept clean to prevent incompatible substances from contaminating the coating. For substrates that are difficult to adhere to, appropriate surface treatments should be applied prior to coating to increase surface energy, improve wetting and leveling, and reduce the occurrence of cratering at the source.

II. Improving Cellulite

Orange‑peel defects manifest as an uneven, bumpy texture on the coating surface, resembling the skin of an orange. They are prone to occur when the leveling time is insufficient or when the coating viscosity is too high. After application, the coating must remain fluid for a certain period to allow surface irregularities to gradually level out. UV resin systems typically cure rapidly; if the leveling time is not properly coordinated with the curing kinetics, the coating can harden before it has had sufficient opportunity to flow, thereby locking in the surface texture and resulting in orange peel. When coating substrates with poor adhesion, the low surface energy of the substrate may inherently limit the coating’s spread and leveling, making the required leveling time even more critical. If the coating viscosity is excessively high, its flowability diminishes, making it difficult to eliminate surface undulations before curing, which increases the likelihood of orange‑peel formation. Although orange peel generally does not compromise the coating’s protective performance, it significantly degrades the aesthetic appearance, often necessitating rework or downgrading in applications that demand high gloss or mirror‑like finishes.

When addressing this issue, formulation design should appropriately control coating viscosity, reducing it while maintaining good application performance to create favorable conditions for leveling. In terms of process control, it is essential to carefully coordinate leveling time with the curing schedule, ensuring that the coating has sufficient time to achieve proper leveling before curing. Additionally, prior to coating, the substrate surface should be treated to increase its surface energy, thereby enhancing the coating’s spreadability and leveling characteristics.

3. Reduce bubble residue

Bubble residues are associated with air entrained during the coating process; if bubbles are not fully eliminated during lamination, they can persist after curing, resulting in surface defects or pinholes. Throughout coating and lamination, processes such as paint agitation, pumping, application, and substrate bonding can all introduce air into the coating film or at the interface. If these bubbles are not expelled or removed prior to curing, they remain trapped within the coating, giving rise to defects. Bubbles located at the coating surface rupture to form pinholes, while those embedded within the coating create closed voids. Bubble residues not only compromise appearance but also disrupt the continuity of the coating in affected areas, thereby reducing its protective performance. In multilayer structures requiring lamination, interfacial bubbles can further impair interlayer adhesion, contributing to inadequate interlayer bonding. The extent to which bubbles can be fully eliminated depends on the coating’s viscosity, the coating application method, and the lamination process: excessively high viscosity hinders bubble escape, and insufficient pressure or overly rapid lamination speeds may prevent interfacial bubbles from being squeezed out, leaving them sealed between layers.

When addressing this issue, formulation design should appropriately control coating viscosity to reduce the resistance to bubble escape from the coating. In terms of process control, during coating and lamination, parameters such as coating speed, lamination pressure, and temperature should be carefully managed to minimize air entrainment and facilitate bubble removal.

IV. Comprehensive Improvement Approach

The causes of surface defects involve multiple factors, including the system’s surface tension, leveling conditions, coating process, and the substrate’s surface characteristics; addressing these issues requires a coordinated approach that integrates formulation design and process control.

In formulation design, adjusting the type and dosage of leveling agents ensures that the coating’s surface tension matches the substrate’s surface energy, while appropriately controlling viscosity to create favorable conditions for wetting, spreading, and leveling without compromising application performance. In process control, maintaining a clean coating environment prevents incompatible substances from contaminating the formulation; carefully managing parameters such as coating speed, nip pressure, and temperature helps minimize air entrainment and facilitates bubble release; and appropriately coordinating leveling time with the curing schedule is essential. For substrates with poor adhesion, performing necessary surface treatments prior to coating can increase surface energy, thereby enhancing wetting and leveling.

V. Conclusion

Surface defects are typical issues that require close attention when applying UV‑curable resins to substrates with poor adhesion, such as automotive interiors and appliance panels. These defects primarily manifest as craters, orange‑peel texture, and trapped air bubbles. Cratering is linked to an imbalance in the system’s surface tension; orange‑peel formation arises from insufficient leveling time or excessive coating viscosity; and residual air bubbles result from entrapped air during application or incomplete removal of bubbles during lamination. To address these defects, a dual‑pronged approach—formulation design and process control—is recommended: adjust leveling agents to optimize surface tension compatibility with the substrate, carefully manage coating viscosity, maintain a clean coating environment, refine coating and lamination process parameters, appropriately balance leveling time with curing kinetics, and ensure thorough substrate surface preparation. Through coordinated improvements in formulation and process control, surface defects can be effectively minimized, enhancing both the aesthetic quality and protective performance of UV‑curable resin coatings on difficult‑to‑adhere substrates.

Disclaimer: The above content has been compiled from publicly available sources and is provided for reference only. If any infringement occurs, please contact us, and we will address it promptly.

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How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 7)

How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 5)